Driver loading previously was very dependent on what code clojure
compiled first. If the Oracle driver was loaded before the plugin
directory was examined, it would result in the Oracle driver never
being loaded. This is due to clojure code only being loaded once when
required and the driver loading code being free floating code that is
only evaled when the namespace is loaded.

This commit switches that code to be in an -init-driver function that
is invoked when the driver is required. This ensures that regardless
of code loading order, the driver will be available if it's present.

This commit also includes code to fail the build if requested to test
on an engine when that engine's driver is not available. Previously if
you set ENGINES to "h2,oracle" but had no Oracle driver in the plugins
directory, the test would succeed, but only test against H2.